1. Field of the Invention
This invention relates to the utilization of solar energy and more particularly, to heat storage materials for use in passive solar heating systems.
2. Description of the Prior Art
For heating particularly in the winter, passive solar heating systems have been recently used in which solar heat is stored in the daytime and the stored heat is utilized for heating in the night. As is well known in the art, heat storage materials for use in the system include sodium sulfate decahydrate, Na.sub.2 SO.sub.4.10H.sub.2 O, and calcium chloride hexahydrate, CaCl.sub.2.6H.sub.2 O. The sulfate decahydrate and the chloride hexahydrate have melting points of 32.degree. C. and 29.degree. C., respectively. These materials are now applied only to passive solar heating systems. In order to enable one to use heat storage materials not only in passive solar heating systems but also in other various ways, it is highly desirable to lower the melting point below 25.degree. C.
In recent years, many studies have been made for saving energy required in greenhouses or vinyl houses. To this end, latent heat storage materials are utilized to store therein solar heat during the daytime so that the stored heat is allowed to release at night for heating the facilities. By the utilization, petroleum which has hitherto been used for heating can be saved to a substantial extent. In ordinary greenhouses which are closed, the indoor temperature in the winter reaches 40.degree. C. or higher during the day in fine weather. In view of the adverse influence on crops, it is customary to ventilate the greenhouse from time to time so that the highest temperature does not exceed 30.degree. C. In order to efficiently accomplish heat storage using as a heat source the air in the greenhouse, which is maintained at a temperature of about 30.degree. C. during a period of 7 to 8 hours in the day, it is necessary that a difference of at least 5.degree. C. be maintained between the temperature of the indoor air serving as a heat source and the melting point of a latent heat storage material. Accordingly, there is a demand for the development of a latent heat storage material whose melting point is below 25.degree. C.